1. Field of the Invention
The present invention relates to a microbench, which is used to optically couple an optical fiber and an optical semiconductor device, and a manufacturing method therefor, and an optical semiconductor module, which is constituted using same.
2. Description of the Related Art
A substrate having V-groove thereon, such as that shown in FIG. 11, has been used as a self-alignment mechanism for the optical coupling of an optical fiber to an optical fiber, or an optical fiber to an optical semiconductor device (Japanese Patent Laid-open No. H5-21817). In the figure, 1 is a silicon (Si) substrate, 2 is back surface metallization, 3 are metallized interconnects, 4 is silica insulation, 5 is a AuSn deposited die area, 6 is an alignment mark, 7 is a V-groove, and 8 is a dicing line. Actually, as long as the substrate has a groove shape, it is possible to affix an optical fiber to the substrate, and a trapezoid or rectangular groove would also be capable of fully performing this function, but because V-shaped processing has been accurate in the past, a V-groove 7 has come to be used most often. Ceramics substrates and Si substrates 1 have been used as this substrate having a V-groove (Japanese Patent Laid-open No. 7-174941). The former has primarily been processed a dicing process, and the latter has been processed via a dicing process, and anisotropic etching with potassium hydroxide (KOH).
A substrate, which has this V-groove 7, and optically couples an optical semiconductor device and an optical fiber in a compact fashion by the V-groove 7 is called a microbench. Among these microbenches, those that make use of Si are called Si microbenches, and to date there have been numerous inventions that put an Si microbench into practice (Refer to xe2x80x9cPackaging Technology in Lightwave Communicationxe2x80x9d Japan Institute of Electronics Packaging Society publication Vol. 1, No. 2, 1998).
A semiconductor laser device (laser diode (LD)) will be used as a typical example of an optical semiconductor device, and will be explained. An LD is lined up with an alignment mark on an Si microbench, and is die bonded, using an AuSn or other soldering material, to a junction down package, which provides light-emitting, light-receiving layers on the substrate side. The facet of an optical fiber is either used as-is, or is used by processing the end into a lens to enhance optical coupling efficiency. Further, there are also cases in which a non-reflective coating is applied to avoid the noise of mode disturbance resulting from reflected light. This optical fiber is affixed in a groove with either plastic or soldering material. With such a simple process, it has become possible to eliminate the time consuming, and costly laser beam welding-based aligning process that had been deemed necessary to date (xe2x80x9cPackaging Technology in Lightwave Communicationxe2x80x9d Japan Institute of Electronics Packaging Society publication Vol. 1, No. 2, 1998).
This Si microbench is manufactured using the following process. (1) A photomask is manufactured using sub-micron order precision. The shape of this photomask is designed so that the center point of an optical fiber, which is a cylindrical shape, is on the substrate surface because of taking into account the KOH-based anisotropic etching of Si. (2) After etching, a dicing line is formed so as to form an optical semiconductor device mounting portion that is orthogonal to the groove. This is because in anisotropic etching, the vicinity of the LD mounting portion is etched in the shape of a triangular pole, and in the state following etching, the distance between the LD and the optical fiber cannot be made shorter, and the purpose is for the dicing line to bring the optical fiber right near the semiconductor device. (3) Since the Si substrate is conductive, silica glass is formed as an insulating layer on parts, which are to be electrically isolated from the semiconductor device, and thereafter, metallization is performed for the interconnects. (4) Metallization and the vapor deposition of an AuSn or other solder are carried out on the semiconductor device mounting portion (die bonding area).
An optical semiconductor module is manufactured by combining the skeletal structure, resulting from the Si microbench, LD and optical fiber, together with ceramics, a leadframe, and a plastic body, and forming an electrical connection (Refer to Japanese Patent Laid-open No. H9-223806, and Japanese Patent Laid-open No. H10-200155).
In the field of optical communications, there has been a tendency to develop large-capacity, high-speed communications in line with the popularization of the Internet and other means of multimedia communications. However, in general, there is a tendency for noise to increase when the telecommunications rate is raised, and a more excellent signal-to-noise (S/N) ratio than in the past is required for telecommunications systems. Meanwhile, lowering the junction temperature of a LD and enhancing light output has the advantage of improving the S/N ratio of a signal. For this reason, there is a need for a semiconductor system with good heat-radiating characteristics,,and a high light output LD device that makes use the system.
In the meantime, in the field of the information industry, even higher speed, higher density recording/playback capabilities are needed. In general, a blue or other such short wavelength LD required in a high-density system still has low light emitting efficiency, necessitating the improvement of the S/N ratio here as well, and to enhance the light output of such LD, there is a need for a high light output LD device that makes use of a system having good heat-radiating characteristics. In particular, even higher light output is required in a recording system than in a playback system.
High speed is a performance capability demanded of an LD. Operating an LD at the high speed of 10 gigabits per second (Gbps) will make it possible to increase capacity four-fold compared to the conventional 2.5 Gbps. For this reason, the capacitance between the substrate and the interconnects, and the inductance of the interconnects themselves, as well as interconnect resistance must be lowered. Because silica is utilized in the insulating film for a Si microbench, the thickness of the film cannot be increased. The problem is that reducing inductance increases the capacitance of the interconnects. According to the disclosure in The Proceedings of the 1995 Electronics Society Conference of IEICE on the Characteristic of High Speed Electrical Circuit on PLC-Platform, on a Study on a Compact Package for Multichannel Multigigabit Optical Interconnection C179, with the object of reducing interconnect capacitance, a thick polyimide sheet was purposely formed on top of an Si microbench, and metallization for the interconnects was formed thereupon. However, in this method, in addition to the fact that polyimide formation had to be carried out with precision thickness, the polyimide foamed during the die bonding of the semiconductor device due to the pressure of absorbed water and caused open circuits and the like, deteriorating the yield of optical semiconductor modules.
Further, because the LD discussed here is a mass production type, which is utilized in ordinary homes, costs must also be lowered. The most costly part of a LD is the Si microbench. In a case in which anisotropic etching was performed on an Si substrate, the situation was such that etching unevenness occurred on the inner surface, making it almost impossible to achieve an uniform anisotropic etching surface. This was a problem in that the optical coupling of the optical fiber and LD could not be accomplished satisfactorily. In Japanese Patent Application Laid-open No. H9-90173, there is disclosed a microbench, which is constituted by forming a plastic using a metal, or a metal and ceramics as filler to reduce the costs thereof. The problem here is that in a case in which a metal was used, interconnect capacitance increased, a polyimide sheet process became necessary, and the yield of optical semiconductor modules deteriorated, in the end, leading to increased costs. Further, in a case in which a plastic was used, in addition to the inherent problem of the [plastic] itself foaming for the reasons explained hereinabove, the problem was that thermal conductivity worsened, causing the S/N ratio to deteriorate.
An object of the present invention is to manufacture and supply a microbench for realizing a highspeed, low-cost optical semiconductor module with a high S/N ratio.
To solve for the above-mentioned problems, the present invention comprises the constitutions described hereinbelow.
(1) A microbench for use in mounting an optical fiber, comprising a substrate made of a ceramics having a specific resistance of 109 xcexa9cm or larger and a dielectric constant of 15 or less, wherein a groove for mounting an optical fiber is formed on the surface of the substrate, there is provided a semiconductor device mounting portion onto which an optical semiconductor device is mounted at the end portion of the groove, and an alignment mark for semiconductor device mounting is disposed in the semiconductor device mounting portion.
(2) The microbench of the disclosure in the above-mentioned (1), wherein said ceramics is either AlN or one having AlN as its principal component.
(3) The microbench of the disclosures in either the above-mentioned (1) or (2), wherein a shape of said groove is either V-shaped or U-shaped.
(4) The microbench of the disclosures in either the above-mentioned (1) or (2), wherein a metallic layer with a thickness of not less than 0.1 mm and not more than 2 mm is disposed on either one surface or both surfaces of said groove and said semiconductor device mounting portion.
(5) The microbench of the disclosure in the above-mentioned (4), wherein, when said metallic layer is disposed on the groove of the substrate, a shape of the groove is unrestricted, and the surface of the metallic layer has a shape of either a V or a U.
(6) The microbench of the disclosure in the above-mentioned (4), wherein, when said metallic layer is disposed in the semiconductor device mounting portion of the substrate, the alignment mark on top of the substrate is omitted, and an alignment mark is disposed on the surface of the metallic layer instead.
(7) The microbench of one of the disclosures of the above-mentioned (4) through (6), wherein a metal of said metallic layer comprises Al as its principal component.
(8) The microbench of one of the disclosures of the above-mentioned (4) through (6), wherein a metal of said metallic layer comprises Cu as its principal component.
(9) A microbench producing method comprising:
providing a baked ceramics substrate;
placing a die on the baked ceramics substrate, the die having configurations, wherein at least one shapes selected from a group of a groove for mounting an optical fiber, a semiconductor device mounting portion and an alignment mark is inverted; and
re-baking the baked ceramics substrate at 1,400xc2x0 C. or higher.
(10) The microbench producing method of the disclosure in the above-mentioned (9), wherein said baked ceramics substrate comprises AlN, said die is placed thereon under a load applied thereto, and said baked ceramics substrate is re-baked in a non-oxidizing environment.
(11) A microbench producing method comprising:
preparing a ceramics sheet in which holes have been made in an optical fiber mounting portion and another ceramics sheet in which there are no holes;
baking both ceramics sheets placed one on top of the other to prepare a ceramics having a groove;
metallizing the surface thereof to form a metalic layer;
inserting a metal chip into either said groove or a portion which will serve as a semiconductor device mounting portion, or both; and
heating at a temperature of 150xc2x0 C. or higher, while at the same time impressing the ceramics, so that an optical fiber mounting portion, a semiconductor mounting portion, and an alignment mark are formed in the ceramics.
(12) The microbench producing method of the disclosure in the above-mentioned (11), wherein said ceramics sheet has AlN as its principal component, and the baking is performed at a temperature of 1,400xc2x0 C. or higher.
(13) An optical semiconductor module, wherein a microbench of one of the disclosures of the above-mentioned (1) through (8) is used, an optical fiber and either a light receiving or a light emitting semiconductor device are mounted and affixed thereto using either a solder or a resin, and thereafter, are electrically connected to metallized wiring on top of said microbench, and the microbench is mounted into either a ceramics package, or a package comprising a leadframe.
FIG. 1 is an illustration of one example of the present invention, and a microbench is supplied by using for the microbench substrate 1xe2x80x2 an AlN substrate, which shows high insulation capabilities, high radiation capabilities and low costs, and has a specific resistance of 109 xcexa9cm or larger, and a dielectric constant of 15 or less.
In a ceramics substrate with a specific resistance of 109 xcexa9cm or larger, and a dielectric constant of 15 or less, the metallized interconnects applied thereon excel in high frequency characteristics. In the ceramics substrate with a specific resistance of 109 xcexa9cm or larger, power leakage from interconnects can be neglected. Thus, for this ceramics substrate 1xe2x80x2, it is not necessary to form a silica insulation layer 4 as with a conventional Si substrate. When the silica on a Si substrate is made thick, the silica layer can peel off and/or cracks can occur, and as such, it is necessary to keep this silica layer thickness to less than 2 xcexcm as far as possible. For this ceramics substrate, it is possible to process same with a thickness of 2 mm or more, enabling the substrate to be made around 1,000-times thicker than the silica of the insulation layer. Since the dielectric constant of the silica is around 3, in the ceramics substrate with a dielectric constant of 15 or less, the width of interconnects in a high-frequency impedance-matching circuit, like a microstrip line, can be steadily widened, thus making it possible to reduce interconnect resistance. Further, stray capacitance was generated from the bonding pads of wires, causing high-frequency disturbance, but this, too, can be kept small by making the substrate thick. Thus, it is possible to prevent the deterioration of the S/N ratio resulting from high-frequency disturbances.
The thermal conductivity of the AlN ceramics substrate, which also satisfies the above-mentioned characteristics, is 170 W/mk or higher compared to 140 W/mk for Si, and the AlN ceramics substrate has much higher heat-radiating capabilities. Thus, it is possible to enhance light output by lowering the junction temperature of an LD. Furthermore, the dielectric loss tangent (tanxcex4), which indicates power loss, is less than 10xe2x88x923 for the AlN ceramics substrate compared to around 10xe2x88x922 for a polyimide film, meaning that the AlN ceramics substrate has good high frequency transfer characteristics, and that the S/N ratio can be improved based thereon.
The shape of the groove for mounting an optical fiber can be either a V-shape or a U-shape, but in a case in which a metallic layer is disposed in the groove, as long as the surface of the metallic layer is either a V-shape or a U-shape, the shape of the groove is unrestricted.
A metallic layer of not less than 0.1 mm and not more than 2 mm thickness is provided on either one surface or both surfaces of the groove and the semiconductor device mounting portion, but this is to shape the groove by using the plastic deformation of the metal, and processing is performed after providing a metallic layer thinly when there is a shape formed on the substrate itself, and thickly when there is no particular shape thereon.
Accordingly, the thickness of the metallic layer is made to range between 0.1 mm and 2 mm. As the material, a metal including Al or Cu as the principal component is appropriate. FIG. 2 is a diagram in which a circular cylinder/optical fiber is mounted in a V-groove, and FIG. 3 is a diagram in which metallization is applied to a V-groove, and a circular cylinder/optical fiber is mounted therein. FIG. 4 is a diagram in which Al is filled into a groove, a U-groove is formed therein, and a circular cylinder/optical fiber is mounted therein, and FIG. 5 is a similar diagram in which Cu is filled into the groove.
An alignment mark for mounting a semiconductor is disposed in an optical semiconductor device mounting portion, but in a case in which the above-mentioned metallic layer is provided, the alignment mark on the ceramic substrate is omitted, and an alignment mark is provided on the surface of the metallic layer instead. FIG. 6 is an example in which alignment mark 6 is provided on substrate 1xe2x80x2, and metallic layer 9 is provided as-is, and FIG. 7 is an example in which alignment mark 6 is omitted on substrate 1xe2x80x2, and alignment mark 6 is provided on metallic layer 9.
To couple optically an optical device with an optical fiber on a microbench, positioning accuracy must be submicron-3 xcexcm. This is because the core diameter of an optical fiber is 10 xcexcm, and approximately ⅓ or more of LD light is incident on the optical fiber. Positioning accuracy in the height direction is only necessary for the distance between the die bonding area and the center of a facet of an optical fiber circumscribed by the groove, and the permissible tolerance is submicron-3 xcexcm for this as well. Because an alignment mark can be aligned with the light emitting/receiving parts of an optical semiconductor device at a tolerance of submicron or less, positioning accuracy in the horizontal direction is only necessary for the distance from the alignment mark to the center of a facet of an optical fiber circumscribed by the groove, and this permissible tolerance is also submicron-3 xcexcm. Since the angle of the light that can be incident in an optical fiber can be up to around +xe2x88x9210 degrees from a vertical line drawn to a facet of the optical fiber, the gradient of the groove and die bonding area must be held to within +xe2x88x923 degrees. Accordingly, a method in which a super hard metal die is used for microprocessing is appropriate. The super hard metal die is a metal die that was processed having Mo or WC as a raw material, and is capable of three-dimensional molding at tolerances of submicron or less.
The following 2 kinds of molding methods, which utilize this metal die, can be conceived.
One is a method in which a metal die is pressed directly against the ceramics substrate after baking in a high-temperature environment, and the groove is processed by deforming the ceramics substrate itself. FIG. 8 shows a photograph of a ceramics surface after processing. It is clear from this photograph that the surface finish is smooth. In this method, a temperature of 1,400xc2x0 C. or higher is necessary in an oxygen-purged nitrogen environment, and thereafter, producing a module can be performed easily by simply metallizing the interconnects and back surface.
The other one is a method in which the metal die is pressed against the ceramics substrate after bonding a Cu or Al metallic chip susceptible to plastic deformation in a location where deformation is necessary. In this producing method, there is the addition of a process for bonding a metallic chip, but molding is possible from ordinary temperature to around 300xc2x0 C. Here, the Young""s modulus of Al is high, and high processing precision is required of the super hard metal die. Further, the coefficient of thermal expansion is a large 23xc3x9710xe2x88x926/degC, and the thickness of the Al layer cannot be made thick due to the unconformability of thermal expansion, but this method is advantageous in that producing is facilitated since Al is susceptible to plastic deformation even at low temperatures approaching ordinary temperature. Since the Young""s modulus of Cu is low, super hard metal die processing is easy, but processing must be done at a high temperature of over 250xc2x0 C., where plastic deformation readily occurs. The coefficient of thermal expansion of Cu is a small 18xc3x9710xe2x88x926/degC, and in addition to the unconformability of thermal expansion being smaller than that for aluminum, it is electrically superior, having electric conductivity that is around 1.5-times that of Al.
As a formation for the groove, when baking an AlN ceramics, it is possible to make a rectangular parallelepiped groove, which is one size larger than an optical fiber, and into which a metallic chip is inserted beforehand. The substrate outfitted with this large rectangular parallelepiped groove can be manufactured by laminating a ceramics temporary compact sheet with holes therein and a ceramics temporary compact sheet with no holes, and baking same. When molding is performed after bonding an Al or other metallic portion thereupon, high-precision groove can be easily processed. The groove cross-section at this time constitutes a 2-layer structure of ceramics and metal.
During these processes, high precision submicron positioning accuracy becomes possible by building into the metal die in advance the die bonding area (the semiconductor device mounting portion), the alignment mark, and the groove. Sometimes a curvature will be formed at the end face of the groove at the part, where the die bonding area and the groove make contact, and the precision for accurately installing an optical fiber worsens. Accordingly, this part is removed by dicing.
In a case in which an optical module is manufactured using an AlN microbench, there are cases in which a leadframe is used. To achieve strength of adhesive bonding of the leadframe and Al wires at this time, Al spot vapor deposition is utilized. When manufacturing the AlN microbench, the AlN substrate, which has the groove that is one size larger than an optical fiber, and a leadframe are bonded, and thereafter, using Al spot vapor deposition, the Al portion can be simultaneously formed on both a wire bonding pad and the groove, prior to die molding. Using this method enables costs to be reduced.
Using this AlN microbench to produce an optical module enables the high yield manufacture of a highspeed, low-cost optical semiconductor module with a high S/N ratio.
In particular, producing an optical semiconductor module using the AlN microbench, which utilizes Al vapor deposition and a leadframe, makes it possible to produce an optical semiconductor module at yet lower costs by using Al wires.